1. Field of the Invention
The present invention relates to a laser heat treatment apparatus and its optical system for forming a polycrystalline semiconductor film having excellent crystallinity to realize a thin film transistor with high mobility.
2. Related Art
A pixel portion of a liquid crystal panel makes up an image by switching the thin film transistors made from an amorphous or polycrystalline silicon film formed on a substrate of glass or composite quartz. Conventionally, a driver circuit for driving these pixel transistors is disposed separately, but it is expected that the driver circuit will be constructed in proximity to the pixel transistor at the same time the pixel transistors are formed. If this is possible, there is a remarkable merit in terms of the manufacturing cost and reliability of the liquid crystal panel.
However, due to low crystallinity of the silicon film constituting an active layer of transistors, the thin film transistors are low in performance, typically in mobility. Therefore, it was difficult to fabricate an integrated circuit requiring high speed and high functionality.
Thus, to realize the thin film transistor with high mobility, a method for improving the crystallinity of silicon film involves a laser heat treatment of radiating a laser to the silicon film formed on the substrate to enhance the crystallinity.
The relationship between the crystallinity of silicon film and the mobility of thin film transistor is described below. The silicon film produced through the laser heat treatment is typically polycrystalline. A grain boundary of polycrystal is constituted of lattice defects, which scatter the carriers in the active layer of the thin film transistor to impede the movement. Accordingly, to enhance the mobility of the thin film transistor, it is important to reduce the number of carriers traversing the grain boundary in moving through the active layer. Therefore, it is required to decrease the density of lattice defects. The laser heat treatment is aimed at forming the polycrystalline silicon film having large crystal grain diameter and with less lattice defects on the grain boundary.
The present inventors proposed an optical system for laser heat treatment in the Japanese Patent Application No. Hei 11-179233, in which a laser beam a laser oscillator is distributed linearly on the surface of silicon film through the optical system, and the linear beam is swept relatively on the silicon film in its orthogonal direction.
FIGS. 7A to 7D are typical views of the optical system. A laser beam 2 radiated from the laser oscillator 1 is passed through intensity distribution forming member 30 and beam shape forming member 31 to illuminate the silicon film on the substrate.
The laser beam 2 radiated from the laser oscillator 1 typically shows a Gauss intensity distribution. The intensity distribution forming member 30 preserves the Gauss intensity distribution in the x direction of beam section, and smoothes the intensity distribution only in the y direction of beam section. The laser beam with such a top hat distribution is adjusted in the magnification of laser beam length in the x and y directions by the beam shape forming member 31, and the beam shape is made rectangular on the amorphous or polycrystalline silicon film 5. If the longitudinal direction of rectangular laser beam is taken as the y direction, the intensity distribution XC in the x direction on the upper face C of the silicon film has a reduced shape of the intensity distribution XA in the x direction on a plane of incidence A for the intensity distribution shaping member 30, still preserving a property of directivity of the oscillating laser beam 2, while the intensity distribution YC in the y direction is substantially uniform on the upper face C of silicon film.
On the other hand, the silicon film as the subject of laser radiation is formed on a silicon oxide film as an under-layer film 6 on the substrate 7 made of glass, which is fixed on a scanning stage, and heated by radiation of the laser beam while the radiated laser beam of rectangular shape is being moved in the x direction.
If the laser beam of rectangular shape is radiated onto the surface of the silicon film 5 formed on the substrate while sweeping it in the x direction, the silicon film 5 is heated by absorbing the laser beam, and melted in a rectangular shape. At this time, there is no temperature gradient in the longitudinal direction or the y direction of radiating laser beam, because the intensity distribution of laser beam 2 is uniform, but there is some temperature gradient caused by cooling in the sweep direction or x direction. When the melted silicon film is crystallized by the cooling, the crystal grows in accordance with the temperature gradient, causing one dimensional growth (one directional growth) in the movement direction on the x direction of the substrate 7, so that the crystal grains having a grain diameter of about several xcexcm are formed along the sweeping direction.
If the rectangular beam is employed for the laser heat treatment, the intensity distribution in the width direction of laser beam has a great effect on a recrystallizing growth process, and the intensity distribution in the longitudinal direction governs the area where the crystal grows, whereby an appropriate beam profile and the intensity of radiation must be chosen to fabricate the thin film transistor having excellent characteristics. However, the optical system for forming the rectangular beam has a limited laser output, and to obtain a required intensity of radiation, it is necessary to shorten the length of radiating laser beam in the longitudinal direction, resulting in lower productivity.
The present invention is achieved in the light of the above-mentioned problems, and it is an object of the invention to provide a laser annealing apparatus that can form a silicon thin film having a smaller density of lattice defect and a high crystallinity through the laser heat process, with a high productivity of the silicon thin film.
The present invention provides a laser optical system for laser annealing comprising a plurality of linear beam forming member for forming the laser beams radiated from a plurality of laser oscillators into linearly radiating laser beams on the surface of an amorphous or polycrystalline semiconductor film formed on a substrate, wherein the laser optical system has each laser oscillator and linear beam forming member corresponding thereto, which are arranged so that an optical axis of laser beam may be substantially parallel to the substrate and in a transverse direction, and has a reflecting mirror for reflecting each laser beam to be applied substantially vertically on the surface of the semiconductor film on the substrate.
In this invention, the reflecting mirror may be a single reflector common to the optical axes of a plurality of laser beams. Thereby, the number of optical parts can be reduced and the optical system simplified.
Also, the linear beam forming member may comprise a cylindrical lens converging in a direction orthogonal to the optical axes of laser beams, in which the cylindrical lens may be a single cylindrical lens common to the plurality of laser beams. Thereby, the laser optical system can be simplified.
Particularly, the laser optical system of this invention may have such a structure that the optical axes of laser beams are arrayed in a longitudinal direction to the surface of the substrate. Thereby, the plurality of laser oscillators, the plurality of linear beam forming member and the plurality of reflecting mirrors are arranged to be piled up in the longitudinal direction.
The laser optical system of this invention can array the linearly radiating laser beams on the surface of the semiconductor film on the substrate to be a substantially straight line in the longitudinal direction of beams. In another form, the laser optical system of the invention can array the radiating laser beams in proximity on the surface of the semiconductor film on the substrate without overlapping each other. With these arrangements, the plural radiating laser beams synthesized on the semiconductor film have an even intensity distribution, thereby widening an area of laser beam radiation to crystallize the wider area through one scanning pass.
The invention is applicable to a silicon film formed on the substrate as the semiconductor film, whereby the polycrystalline silicon substrate can be used for the thin film transistor having excellent crystallinity.
The laser oscillator is preferably a pulse laser oscillator having an oscillation wavelength from 330 to 800 nm, and particularly uses the harmonics of a solid state laser. Laser beams in this range of wavelength are especially effective for uniformly heating the amorphous silicon film in the thickness direction.
Moreover, the invention provides a laser annealing apparatus comprising the laser optical system combined with a stage for laying the substrate formed with the semiconductor film, wherein a relative movement between a linear laser beam and the stage is made by moving the laser beam in its width direction on the surface of the silicon film to melt and crystallize the silicon film. Preferably, the stage is provided with a scanning drive device to enable the stage to be scanned.